The present invention relates generally to a method and corresponding apparatus for automatically actuating a vehicle wheel brake with the aim of preventing the wheel from skidding in conjunction with braking and acceleration. More specifically, the present invention relates to such a method wherein the instantaneous speed of rotation of the wheel is sensed and the instantaneous absolute speed variation is calculated, after which the wheel brake is automatically released or actuated depending upon the sensed conditions.
According to the aforementioned brake actuation method, the wheel brake is automatically released during braking of the vehicle when the speed of rotation of the wheel reduces so that its absolute speed variation is greater than a predetermined maximum permissible value. The wheel brake is also automatically released during braking when the speed of rotation of the wheel approaches zero and the vehicle is still in motion. The wheel brake is applied automatically during vehicle braking when the speed of rotation of the wheel has increased to a predetermined reducing value after the brake has previously been released.
During vehicle acceleration, the wheel brake is applied automatically when the speed of rotation of the wheel increases so that its absolute speed variation is greater than the aforementioned predetermined maximum permissible value. The wheel brake is released automatically during acceleration of the vehicle when the speed of rotation of the wheel has fallen to a predetermined increasing value after the brake has been applied.
The wheel brake of the aforementioned brake actuation method remains unaffected and is not subject to automatic actuation when the absolute variation in the speed of rotation of the wheel is less than the aforementioned predetermined maximum permissible value. Also, the wheel brake is applied on braking once a predetermined period has elapsed since the brake was released, and is released on acceleration once a predetermined period has elapsed since the brake was applied.
U.S. Pat. No. 3,917,353 contains an exhaustive description of the above-mentioned method and a system which operates in accordance therewith, including discussion of the difficulties which are associated with braking. A system which proposes to resolve similar problems during both braking and acceleration is described in U.S. Pat. No. 3,584,921.
U.S. Pat. No. 3,958,834 discloses the re-application of a brake during braking once a predetermined period has elapsed since the brake was released automatically, without the unbraked wheel having previously reached an acceleration threshold.
While the above-mentioned system operates satisfactorily when the friction between the road surface and the wheel remains substantially constant, i.e., the road surface is either slippery or coarse, it has been found that the required braking distance for such a system becomes unnecessarily long under variable road conditions, due to an inability of the system to take account of and provide effective braking under such variable conditions.